Samples of the alpine and subalpine snowpack were collected in and near the headwater basins of the Yampa River in northwestern Colorado during maximum annual accumulation of snowpack in spring 1995. Sampling protocol at seven selected sites at more than 2,500 meters above sea level divided the snowpack into two distinct strata to enable separate chemical analyses of upper and lower layers of the annual snow cover. These two layers correspond to the early snow season beginning in September until December 12, 1994, and the remainder of the season from January 1 until the sampling date in spring 1995. At one site these two strata were resampled at closely spaced intervals defining substrata to observe variance within the two strata dividing the snow season. Analytical results of snowpack chemistry are presented in support of investigations of seasonal effects on ion concentrations in the snowpack. Chemical concentrations of major anions and cations, dissolved organic carbon, and alkalinity; measured pH; calculated charge balance between anions and cations; the stable-sulfur isotope ratio (34S/32S); and summary statistics of chemical concentrations are tabulated. Sampling sites are plotted on a map of the area. Spatial distributions of the concentrations of the hydrogen, nitrate, and sulfate ions and stable- sulfur isotope ratios also are mapped. Several unusual, late-season, snowfall events occurred during April and May of 1995 after the snowpack was sampled at most of the seven sites in the study area. Consequently, a considerable fraction of the total annual snowpack was not sampled. At one site, the full snowpack was sampled again in June, after the late-spring storms, for comparison to the chemistry of the snowpack sampled earlier in April. Precipitation chemistry from a National Atmospheric Deposition Program (NADP) collector located near the site is presented for comparison of the chemistry of the late-season snow to that of the typical annual snowpack season.